1. Field of the Invention
The present invention relates to a projection-type display apparatus, and more particularly to a projection-type display apparatus provided with a reflection-type illumination optical system as illumination with which to project an image.
2. Description of the Prior Art
One conventionally known example of a reflection-type display panel for use in a projection-type display apparatus is a Digital Micromirror Device™. The Digital Micromirror Device is a binary spatial light modulator developed by Texas Instruments. The Digital Micromirror Device is provided with a plurality of micromirrors of which each is rotatable about an axis that is slanted at a predetermined angle (for example, 45°) relative to the longer or shorter sides thereof. Each pixel is turned on and off by bringing the corresponding micromirror into one of two states with different inclination angles (for example, ±12°). When a micromirror is in an ON state, the illumination light reflected therefrom is directed to the entrance pupil of a projection optical system and then reaches a screen. By contrast, when the micromirror is in an OFF state, the illumination light reflected therefrom is directed in a direction deviated from the entrance pupil of the projection optical system, and thus produces no image on the screen.
In a projection-type display apparatus incorporating a Digital Micromirror Device, which operates as described above, the entire image presented on the Digital Micromirror Device needs to be illuminated from an oblique direction so that the illumination light reflected from micromirrors in their ON state is efficiently directed to the entrance pupil of a projection optical system. Simultaneously, the illumination light reflected from micromirrors in their OFF state needs to be kept from entering the entrance pupil of the projection optical system. To achieve this, various types of illumination optical system have been conventionally proposed. For example, U.S. Pat. No. 6,129,437 proposes an illumination optical system provided with an elliptical mirror and a condenser lens.
Using a lens in an illumination optical system results in chromatic aberration, and correcting it requires an increased number of components. As compared with a lens surface, a reflective surface can be given a stronger optical power per surface, and therefore using a reflective surface in an illumination optical system helps not only to reduce the number of components but also to prevent chromatic aberration. However, using a reflective surface in combination with a lens surface as practiced in the illumination optical system proposed in the aforementioned patent publication makes it difficult to obtain satisfactory optical performance as expected in an illumination optical system, and thus leads to an unduly expensive and large display apparatus.